Generally, compression molded articles using a sheet molding compound (hereinafter referred to as "SMC" for short) as the molding material enjoy high strength and rigidity and yet suffer from deficiency in surface property and production cycle. In contrast, injection molded articles using a bulk molding compound (hereinafter referred to as "BMC" for short) exhibit satisfactory surface property and enjoy a short production cycle and yet suffer from deficiency in mechanical strength. Although the injection molding method using the BMC has long been in practice, the products thereof are mostly small molded articles represented by electrical parts. It is rarely employed for the production of large shaped articles solely because the products thereof are deficient in properties of strength, particularly impact strength.
The BMC to be used for injection molding has heretofore been produced by causing chopped glass fiber strands to be impregnated with an unsaturated polyester resin composition by the use of a kneading machine such as a twin-arm kneader, for example. Since the glass fiber strands are opened, bent, and injured by the mechanical shearing force exerted thereon during the course of manufacture of BMC, the BMC, even in the compression molded form, in which the glass fiber strands are not damaged further in the course of molding, has only about one half of the impact strength of the SMC at most. Further, even during the course of injection or transfer molding, comprising the steps of feeding the BMC to a stuffer, forcing it into a plunger or a screw, conveying, metering, and injecting a compressed mass of the BMC, advancing this mass of the BMC through the interior of a die, and curing the discharged the BMC mass, the glass fiber strands in the BMC are conspicuously opened and bent and injured all the more by the mechanical force exerted on the BMC. The finished products obtained by injection or transfer molding the BMC, therefore, have very low degree of impact strength, even below one fourth of those of the products obtained by compression molding the SMC.
Various devices have been heretofore invented such as for shortening the time of blending as by optimizing the blending machine, for example (U.S. Pat. No. 3,932,980), for repressing the susceptibility of glass fiber strands to the detrimental phenomena of opening and injuring as by selecting the kind of a binder for glass fibers or regulating the amount of the binder applied thereto, for rendering chopped glass fibers difficult to break or intertwine as by decreasing the length thereof to the neighborhood of 1/4 inch (Japanese patent Laid-Open SHO 57(1982)-41,918), and for preventing glass fibers from being damaged owing to mutual friction by keeping the glass fiber content in the BMC below the level of 20% by weight. They, however, have failed to bring about any appreciable improvement.
An object of this invention, therefore, is to provide a novel fiber-reinforced thermosetting resin molding material and a method for the production thereof.
Another object of this invention is to provide a fiber-reinforced thermosetting resin molding material which retains the advantages such as outstanding surface properties and productivity owned by the conventional BMC, shows great improvements in mechanical properties, particularly in impact strength, and permits production of medium to large shaped articles of high practical value and a method for the production thereof.